The 1995-2001 Lumina/Monte Carlo vehicles use a VATS - Vehicle Anti Theft System. This is composed of a kill switch for the fuel pump/ignition systems and a resistor pellet in your key. Chances are your VATS reader in the ignition cylinder on your steering column has decided to play a game with you. Either have it cleaned or get a dealer to have it recored and coded for you. Of course, this isn't for-sure, but its the closest I can get you to it.